Devices may be configured to induce electromagnetic heating and an eddy current may be generated on an inductive heat-generating layer of a heating member. Eddy currents may be generated by, for example, a magnetic flux generated by an excited coil. When Joule heat generated by the eddy current causes the heating member to generate heat, the heating member is heated to a prescribed fixing temperature. With this type of device, the thermal capacity of the heating member can be reduced to shorten the warm-up time associated with the device. In addition, the device may be equipped with a compact structure to obtain a high heat-exchanging efficiency.
With a fixing device configured to induce electromagnetic heating, heat is often lost from the surface of its heating member when the surface makes contact with paper. In particular, the heating member may have a paper feeding area and a non-paper feeding area (or a “non-feeding area”). Thus, a paper feeding area where paper passes is likely to be at a lower temperature than non-feeding areas where paper does not pass. Paper sheets with smaller sizes may be in a fixed position, particularly when the paper sheets are fixed sequentially or in succession. If the area corresponding to the paper feeding area is maintained at fixing process temperature, the temperature of the non-feeding area may become excessively high. This may cause the temperatures of the heating member and excited coil to exceed their maximum allowable temperatures, and they may be damaged as a result.
A proposed fixing device has a magnetic core with a Curie temperature set to a temperature that is slightly higher than fixing process temperature. The fixing device also has a coil that uses the magnetic core to generate a magnetic flux by which the heating member is inductively heated. The magnetic core has different Curie temperatures in a direction orthogonal to a paper-conveying direction. Specifically, the magnetic core in the fixing device is configured so that the Curie temperature at both ends of the magnetic core is lower than the Curie temperature at the central portion of the magnetic core. When small sheets of paper are fixed in succession, this structure can prevent a large deviation in temperature between the paper feeding area and the non-feeding areas. With this type of fixing device, the Curie temperature at both ends of the magnetic core (also referred to “end-sides of the magnetic core”) is equivalent to that of the non-feeding areas when small sheets of paper are fed. Thus, the Curie temperature at the end-sides of the magnetic core are set to a temperature lower than the Curie temperature of the central portion of magnetic core that includes the paper feeding area for small-sized paper. In some instances, the temperature of the heating member corresponding to the non-feeding area may become excessively high during the fixing of small-sized paper. In such instances, both ends of the magnetic core or the end-sides of the magnetic core may have been heated to or above their Curie temperature due to thermal radiation or thermal conduction from the heating member. In response, the magnetic permeability of the end-sides of the magnetic core may be lowered. As such, lowering the magnetic permeability reduces the amount of heat generated in the area corresponding to the non-feeding areas on the heating member. Therefore, the temperature on the heating member corresponding to the non-feeding areas can be lowered to prevent damage to the fixing device.
A magnetic core in another fixing device has a plurality of first magnetic cores. The magnetic core may be formed in a trapezoidal shape and placed in a direction orthogonal to a paper conveying direction so as to cover a coil that generates a magnetic flux. The magnetic flux may be used for inductive heating and may also have a plurality of second magnetic cores that are placed in clearances or hollow areas formed by rings of a coil. The rings of coil may be wound in a loop shape in a direction orthogonal to the paper conveying direction. The Curie temperature of each end-side of the magnetic core may correspond to one non-feeding area. Further, the Curie temperature of the second magnetic core may be set to a temperature lower than the Curie temperature of the first magnetic core. Each end-side of the magnetic core may be positioned separately from the first magnetic core and may have a lower thermal capacity than the first magnetic core. With this structure, the temperature on areas of the heating member corresponding to the non-feeding area may be controlled and prevented from becoming excessively high. In particular, the temperatures of the end-sides of the magnetic core (e.g., the non-feeding areas) may be controlled to reach their Curie temperature relatively quickly due to the thermal radiation or thermal conduction from the heating member to the end-sides of the magnetic core. Therefore, this prevents the area on the heating member corresponding to the non-feeding area from becoming excessively high.
With the fixing device described above, however, the end-sides of the magnetic core may not adequately track changes in the temperature of the heating member.
If the end-sides of the magnetic core cannot adequately track changes in the temperature of the heating member, a fixing failure may occur because heating in the non-feeding areas may become insufficient. This may happen when small sheets of paper are fed in succession and the temperature of the end-sides of the magnetic core corresponding to the non-feeding areas exceed their Curie temperature. As such, if paper with a larger fixed size is fed, uniform heating and fixing cannot be performed over the entire surface of the paper with the fixed size.